The specification herein incorporates disclosure contained in Disclosure Document No. 433067 which was received in the U.S.P.T.O. on Feb. 25, 1998.
The security inspection of luggage and other containers commonly utilizes two types of x-ray imaging systems generally referred to as the "line scan" and the "flying spot" systems. A line scan system employs an x-ray source and slit collimator to form a fan beam of x-rays directed perpendicular to the conveyor belt. As an inspected item intersects the fan beam, x-rays pass through the item to a multi-element linear array detector positioned below the inspected item in line with the fan beam. Inspected items are most often moved through the fan beam utilizing a moving inspection surface, such as a conveyor belt, with the multi-element detector located below the inspection surface. The array detector collects the transmitted x-rays for processing into transmission x-ray images of the inspected item.
A key feature of the line scan geometry is the use of the multi-element detector to obtain spatial resolution along the scan line. The detectors of some line scan systems are designed to provide separate signals for the low-energy and the high-energy x-rays. The separate signals are used to create separate images of the low and high-energy attenuation properties of the inspected objects. For example, a line scan system having a configuration including both low-energy and high-energy x-rays may be used to discriminate between organic and inorganic materials.
The second common imaging system, the flying spot system, also utilizes an x-ray source having a slit collimator to form a fan beam as in the above described line scan system. However, in the flying spot system, the fan beam is additionally collimated by a second rotating chopper that is positioned in the path of the fan beam between the x-ray source and the target item to be scanned As the chopper rotates, slits in the second chopper permit a pencil beam of x-rays to sweep rapidly across the target item.
The transmission x-ray detector in the flying-spot x-ray system produces a single signal related to the total number of x-rays striking anywhere along its length. Thus, the spatial resolution along the scan line is determined by the controlled movement of the pencil beam of x-rays produced as a result of the instantaneous positioning of the chopper as it rotates. Each pixel in an acquired image corresponds to a small area on the conveyer belt or inspected item that is illuminated by a pencil beam of x-rays. For example, an image composed of 800 pixels along a scan line corresponds to a pencil beam having a nominal width of 1/800th of the scan line length.
A significant advantage of the flying spot system over the line scan system is that it can be used to acquire back-scatter x-ray images and forward-scatter x-ray images which more readily detect materials having low atomic numbers, also referred to as "low Z" materials. Back-scatter and forward-scatter x-rays, collectively referred to as "back-scatter" x-rays, are scattered or reflected from an inspected object, and thus, move in random directions. In contrast, transmitted x-rays are transmitted or partially transmitted through the object being inspected and are focused upon the transmission detectors. The back-scatter detectors utilized by the flying spot system are large area x-ray collectors that do not rely on spatial resolution to form an image. Thus, the random movements of the scattered or reflected x-rays do not present a problem for the flying spot system. In comparison, line scan systems, which utilize transmission detectors that inherently rely on the x-rays being focused to form an image, are unable to acquire back-scatter images.
The advantage of the line scan system over the flying spot system is the more efficient use of x-ray flux. The flying spot system, which employs a rotating chopper, only uses a small fraction of the x-rays present in the fan beam. In contrast, the line scan system utilizes the entire fan beam to produce an image. Because the signal-to-noise ratio (SNR) of an x-ray image is determined by the number of x-rays that are used to create the image, flying spot systems do not provide high quality images. A common approach to improving image quality is to increase the power of the x-ray source to compensate for the inefficient x-ray flux usage. For example, rotating-anode x-ray tubes, special electrical wiring to power the system, and better heat dissipation configurations may be utilized to increase x-ray power. However, these methods produce disadvantages in both cost and complexity.